The long-range goal is to explain how respiratory rhythm in the mammal arises from interactions between brain-stem respiratory neurons. The experimental approach is to record the activity of neurons in regions of the cat brain-stem together with activity in respiratory motor nerves, under the influence of various experimental inputs. Respiratory neurons are classified by: a) anatomical location; b) discharge pattern, both on slow and fast time-scales; c) responses to experimental inputs. The mechanisms of the transitions between the inspiratory (I) and expiratory (E) phases (I yields E-switching and E yields I-switching) will be studied by observing responses of neurons to timed experimental inputs that advance or retard phase-switching: a) lung inflation; b) electrical stimulation in the rostral pontine pneumotaxic center (PC); c) electrical stimulation of vagal (V) and superior laryngeal (SL) afferents. Special emphasis will be placed on identification of possible off-switch neurons by monitoring changes of activity immediately preceding a phase-switch. Short time-scale interactions between pairs of neighboring I or E neurons will be studied by recording from two microelectrodes positioned so that their tips closely approach each other within a local group of respiratory neurons; these will be analyzed by cross-correlation and differential activation of inputs. The neural circuits responsible for the high frequency oscillations (HFO) in discharges of I neurons will be studied by use of timed inputs to reset the HFOs and to produce phase-response curves. The discharge properties of PC neurons, located in nucleus parabrachialis medialis (NPBM) and in the Kolliker-Fuse (KF) nucleus, will be studied in relation to phase-switching inputs. The afferent and efferent connections of PC neurons with medullary respiratory neurons will be ascertained by analysis of evoked responses.